The most prevalent interface for computer graphic displays is an analog video signal composed of three channels, typically a red channel, a green channel and a blue channel. This is commonly referred to as an RGB interface. Conventionally, the video signal is applied to a rasterized analog display device, such as a CRT monitor, to generate an image. Normally, the vertical and horizontal sync pulses, which control the raster scanning of the video signal, are provided separately. However they can be embedded on one of the color channels, which is commonly referred to as "composite sync".
More recently, it has become common to display the image on a digital display device. When the RGB signals are displayed on fixed matrix displays, such as desktop LCD displays, the analog signals are usually sampled and digitized, and the resulting values are assigned to discrete pixel locations in the display matrix. Since the video signal is composed of three separate analog channels, the sampler typically consists of three or more separate channels. For higher resolution signals, sampling clock rates are high enough that a three channel sampler may be too costly. In this case, each video channel is often assigned to two or more sampling channels that operate out of phase with each other.
One of the challenges with the analog-to-digital sampling process is that of achieving inter-channel balance among the three colors. In other words, in order to achieve the intended color and contrast fidelity in the displayed image, all three samplers should generate the same data values for the black level, and the same data values for the white level. This is only possible if the channels have nearly identical DC offsets (black level) and gain (white level). It is also important that all three channels have the same monotonic and linear gray-level response.
In the case of a multi-phase system where each color channel has more than one sampler operating at different respective phases, it is also important that the system maintain intra-channel balance. Failure to do so can introduce image distortions such as striping.
In the past, digital display systems have provided the user with a mechanism to manually control the inter-channel balance, usually in the form of a sub-contrast or sub-color control. Such an implementation usually adjusts the gain of each channel independently. This same control is commonly used to also adjust the white point of the display. In addition to adjusting the channel balance, manual user controls are also normally provided to allow the user to adjust the contrast and brightness of the display. This procedure typically involves adjusting the DC offset of all three channels simultaneously, in the case of brightness, and the gain of all three channels simultaneously, in the case of contrast.
In addition to color balance, other operating parameters of the analog-to-digital conversion process affect the quality of the displayed image. Examples of these include the phase and frequency of the data clock that is used for the conversion of the analog signal and the reading of the digital data into the digital display device. Also of interest is the location of the image, e.g. centering and alignment, on the display.
It is desirable to provide for automatic balancing of three separate color sampling channels. Further along these lines, it is desirable to automatically configure a digital display to achieve the maximum contrast and brightness, as well as automatically control the white point of the display, the centering of the image, and the phase and frequency of the data clock.